In suppressed ion chromatography (“IC”) of anions, aqueous eluent solutions of cation carbonate/bicarbonate and hydroxide herein carbonate/bicarbonate or hydroxide eluents) are most commonly used. Carbonate eluents, which are used primarily in isocratic separations, are suppressed to carbonic acid. Depending on the concentration of the eluent, the conductivity of the suppressed eluent is typical 10-20 μS/cm. The higher background conductivity results in greater noise and reduced analyte intensity (signal) which compromises detection limits. Hydroxide eluents, which are used in both the isocratic and gradient mode, suppress to water, and the background conductivity can be as low as low as 0.2 μS/cm. In practice, background conductivities of suppressed hydroxide are usually in the range of 1-3 μS/cm due to contamination of the eluent with ambient carbon dioxide.
Carbonate eluents typically contain both carbonate (CO32−) and bicarbonate (HCO3−). One of the advantages of carbonate eluents is the ease of controlling the ratio of carbonate to bicarbonate, thereby affecting the selectivity of the separation. Unlike hydroxide eluents, where care must be taken to prevent contamination from ambient carbon dioxide, carbonate-based eluents do not suffer from this problem. Many IC stationary phases have been developed for carbonate eluents which take advantage of the monovalent/divalent nature of carbonate eluents.
With hydroxide-based eluents or water used to electrolytically generate hydroxide, the presence of carbon dioxide in the air causes contamination of the eluent. Carbonate contamination of hydroxide eluents compromises the chromatographic separation. Also, the detector response of analyte anions is affected by the increase background conductivity as the result of carbonate. Samples containing carbonate can interfere with the separation and detection of analytes as well. Thus, there is a need in IC for carbonate removal devices.
Dionex Corporation sells two CO2-removal devices, under the CRD 200 and the CRD 300 tradenames. The CRD 200 is used to remove CO2 present in hydroxide eluents and samples and is placed between the suppressor outlet and the conductivity cell inlet. The CRD 300, which offers greater CO2 removal capacity, is used primarily with carbonate eluents and is also placed between the suppressor outlet and the conductivity cell inlet. The CRD 200 and CRD 300 both use a liquid or gas flow stream on the outside of the fiber to remove the carbonic acid as it diffuses from the inside of the fiber to the outside. This liquid or gas flow (regenerant) prevents the accumulation of carbonic acid on the outside of the fiber membrane. The CRD 200 and CRD 300 use either a pumped external chemical regenerant (base) or a vacuum pump to remove the carbonate. The base regenerant promotes the diffusion of the carbonic acid through the fiber membrane since carbonic acid (carbon dioxide) is readily soluble in base. As the base regenerant passes through the decarbonation chamber, the base becomes contaminated with carbonate and is then diverted to waste. Thus, periodic base regeneration is used in this system.